On Mon, Jul 27, 2020 at 11:45 AM Richard Sandiford <richard.sandif...@arm.com> wrote: > > Richard Biener <richard.guent...@gmail.com> writes: > > On Mon, Jul 27, 2020 at 11:09 AM Richard Sandiford > > <richard.sandif...@arm.com> wrote: > >> > >> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes: > >> > On Wed, Jul 22, 2020 at 5:18 PM Stefan Schulze Frielinghaus via > >> > Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > >> >> > >> >> This is a follow up to commit 5c9669a0e6c respectively discussion > >> >> https://gcc.gnu.org/pipermail/gcc-patches/2020-June/549132.html > >> >> > >> >> In case that an alignment constraint is less than the size of a > >> >> corresponding scalar type, ensure that we advance at least by one > >> >> iteration. For example, on s390x we have for a long double an alignment > >> >> constraint of 8 bytes whereas the size is 16 bytes. Therefore, > >> >> TARGET_ALIGN / DR_SIZE equals zero resulting in an infinite loop which > >> >> can be reproduced by the following MWE: > >> > > >> > But we guard this case with vector_alignment_reachable_p, so we shouldn't > >> > have ended up here and the patch looks bogus. > >> > >> The above sounds like it ought to count as reachable alignment though. > >> If a type requires a lower alignment than its size, then that's even > >> more easily reachable than a type that requires the same alignment as > >> the size. I guess at one extreme, a target alignment of 1 is always > >> reachable. > > > > Well, if the element alignment is 8 but its size is 16 then when presumably > > the desired vector alignment is a multiple of 16 we can never reach it. > > Isn't this the case here? > > If the desired vector alignment (TARGET_ALIGN) is a multiple of 16 then > TARGET_ALIGN / DR_SIZE will be nonzero and the problem the patch is > fixing wouldn't occur. I agree that we might never be able to reach > that alignment if the pointer starts out misaligned by 8 bytes. > > But I think that's why it makes sense for the target to only ask > for 8-byte alignment for vectors too, if it can cope with it. 8-byte > alignment should always be achievable if the scalars are ABI-aligned. > And if the target does ask for only 8-byte alignment, TARGET_ALIGN / > DR_SIZE would be zero and the loop would never progress, which is the > problem that the patch is fixing. > > It would even make sense for the target to ask for 1-byte alignment, > if the target doesn't care about alignment at all.
Hmm, OK. Guess I still think we should detect this somewhere upward and avoid this peeling compute at all. Somehow. Richard. > Thanks, > Richard
